1. Field of the Invention
This invention relates to electronic musical instruments of the tone synthesizer type and in particular is concerned with provision for providing a polyphonic portamento effect in a digitally controlled tone synthesizer.
2. Description of the Prior Art
Portamento is a musical effect which is almost universally incorporated as a feature of electronic musical instruments of the synthesizer variety. Generally the portamento mode is limited to monophonic tone generation which is adequate for most synthesizer tone generators.
Two principal types of portamento system operational modes have been implemented in prior systems. The first mode can be categorized as a continuous tone mode. In the first operational mode, the tone generation starts when a keyboard keyswitch is actuated and the keyboard has been placed in the portamento operational status. The tone continues even when the keyswitch is released and continues to sound a note at the corresponding keyswitch pitch until a new keyswitch is actuated. When a new keyswitch is actuated, the musical pitch of the generated tone gradually changes to that of the new note in a manner that "sounds" like a continuous smooth frequency transition. The frequency transition speed is preset by a variable musical console control. Usually the frequency transition takes place as a constant frequency change per unit time. A variation is to cause the frequency transition speed to be proportional to the frequency change between the old and new frequencies.
The second operational mode is one in which each actuated note has its own complete ADSR envelope modulation. When a new keyboard keyswitch is actuated, a new note will be initialed which has a pitch that changes smoothly from that of the previously operated keyswitch. At the time of the keyswitch actuation, an attack phase of the ADSR envelope is initiated for the new note.
An example of a monophonic portamento system is disclosed in the U.S. Pat. No. 4,103,581 entitled "Constant Speed Portamento."
The current trend in the design of electronic musical instruments is to implement polyphonic tone generators each member of which is a tone synthesizer. Portamento systems designed for monophonic operation are not readily incorporated into polyphonic systems. A system problem arises if the polyphonic musical instrument is implemented by having a few tone generators that are assigned to any of the actuated keyswitches on the musical instrument's keyboard. Tone generator problems arise when portamento is implemented in such instruments because of the necessity of assigning currently unused tone generators to newly actuated keyboard switches.
A problem in polyphonic portamento systems using the assignment of a number of tone generators arises when a chord is played and is then followed by a chord in which the number of component notes differs from that of the first chord. This problem of generator assignment and portamento frequency transition is made further complex if the second set of notes in the new chord are not actuated simultaneously. These conditions can readily result in portamento frequency transitions which can produce objectionable "non-musical" dissonances.
It is an object of this invention to provide means for assigning a set of tone generators operating in a polyphonic portamento mode such that frequency transitions to new notes are made without dissonances produced by frequency cross over transitions.
It is a further object of this invention to automatically reassign tone generators if new notes are actuated before the completion of a portamento frequency transition in a fashion which eliminates the chance of a dissonant frequency cross over.